1. Field of the Invention
The present invention relates to a semiconductor memory device and, particularly, to a magnetic semiconductor memory device having a magnetic memory cell for storing information in accordance with a magnetization direction of a magnetic substance. More specifically, the present invention relates to a configuration for reducing magnetic disturbance at the time of writing data in a magnetic semiconductor memory device.
2. Description of the Background Art
An MRAM (Magnetic Random Access Memory) attracts a strong attention as a memory device capable of storing data in a nonvolatile manner with low power consumption. The MRAM utilizes the characteristics that magnetization generated in a ferromagnetic material by an externally applied magnetic field remains in the ferromagnetic material after removal of the external magnetic field. By changing the magnetization direction of residual magnetization in the ferromagnetic material in accordance with data, data is stored. As data storing elements of memory cells of such an MRAM, a giant magneto-resistance element (GMR element), a colossal magneto-resistance element (CMR element), and a tunneling magneto-resistance element (TMR element) are known.
For the structure of a data storing part of an MRAM cell, two magnetic layers are stacked with an insulating film sandwiched in between. The magnetization direction of one of the two magnetic layers is used as a reference magnetization direction and the magnetization direction of the other magnetic layer is changed according to storage data. Magnetic resistance varies according to match or mismatch of the magnetization directions of the magnetic layers and, accordingly, a current flowing via the storing part varies. By detecting the current flowing via the magnetic layers of the storing part, data is read. In writing data, the magnetization direction of the magnetic layer for storing data is set according to storage data by a magnetic field induced by current flow.
An example of the configuration of such an MRAM is disclosed in, for example, a prior art literature 1 (Japanese Patent Laying-Open No. 2002-170375).
In prior art literature 1, for a data storing element in a memory cell, a TMR element is used. In the TMR element of prior art literature 1, a hard layer of high coercive force and a soft layer of low coercive force are disposed so as to face each other with a tunnel insulation film sandwiched in between. According to the magnetization direction of the hard layer, data “0” or “1” is stored.
In writing data, a current is caused to flow in a predetermined direction through a write line (write word line). The magnetization direction of the soft layer is determined by a magnetic field induced by the current flowing in the write line, while the magnetization direction of the hard layer is not changed by the magnetic field induced by the current flowing in the write line. In this state, a current is caused to flow in the direction according to storage data through a bit line connected to the hard layer. By a combined magnetic field of perpendicularly intersecting magnetic fields induced by currents flowing in the write line and the bit line, the magnetization direction of the hard layer is determined and data is accordingly stored.
Data stored in the TMR element of the prior art literature 1 is read in three stages. First, a current is conducted in a predetermined direction in a write line to set the magnetization direction of the soft layer to a predetermined direction. Subsequently, the TMR element is electrically connected to a ground node via an access transistor. In this state, a read current is conducted to the bit line and a voltage according to the current flowing from the bit line via the TMR element in the memory cell is stored into a first sense node of a sense amplifier. TMR element provides a reduced resistance to cause a large current flow when the hard layer and the soft layer are the same in magnetization direction, while providing a large resistance to cause a reduced current flow when the hard layer and the soft layer are different in magnetization direction from each other. Thus, in this first stage, information according to whether or not the magnetization direction of the hard layer is the same as that of the soft layer is stored in the first sense node of the sense amplifier.
Then, the magnetization direction of the soft layer is inverted by flowing current in the reverse direction through the write line. The TMR element is connected again to the ground node in this state and a voltage according to the current (current flowing from the bit line via the TMR element) is stored into a second sense node of the sense amplifier.
Subsequently, by differentially amplifying the voltages at the first and second sense nodes of the sense amplifier, data is read. Specifically, the amount of current flowing in the bit line when the magnetization direction of the hard layer is the same as the initialization magnetization direction of the soft layer is different from the current amount when the magnetization direction of the hard layer is different from the initialization magnetization direction of the soft layer. Therefore, voltages of different levels are stored at the first and second sense nodes of the sense amplifier. By differentially amplifying the voltages of the first and second sense nodes, data stored in the TMR element is read.
Changing the magnetization direction of the soft layer to the initialization direction and then to the opposite direction is made for the following reason. In data writing, the directions of the current flow through the write line and bit line vary according to the write data. Therefore, the magnetization direction of the soft layer might differ for different write data. Consequently, the magnetization direction of the soft layer is initialized to a predetermined direction to ensure accurately set the magnetization direction of the soft layer in data reading.
In the prior art literature 1, in reading data, to read complementary data to the sense nodes, the magnetization direction of the soft layer is inverted. At the time of inverting the magnetization, the current flowing in the write line is inverted. When a noise induced by inversion of the current in the write line occurs on the bit line and the noise is superimposed on the voltages read on the sense nodes of the sense amplifier, an accurate sensing operation cannot be performed. To prevent erroneous reading of data due to the induction noise on the bit line, in the prior art literature 1, a countermeasure for preventing the induction noise from reaching the sense node in the sense amplifier at the time of inverting the magnetization of the soft layer is taken. For the countermeasure, a countermeasure of setting a bit line in a floating state at the time of inverting the magnetization, a countermeasure of connecting inductance between the sense amplifier and the bit line to reduce induction noise, and a countermeasure of connecting the bit line to the ground node at the time of inverting the magnetization to discharge the induction noise to the ground node are proposed.
In the prior art literature 1, it is considered that, at the time of reading data, the induction noise occurring when the magnetization of the soft layer is inverted is prevented from exerting an adverse influence on the data reading. However, the prior art literature 1 does not consider an influence of the magnetic field, induced by the currents flowing in the write line and the bit line at the time of writing data, on the TMR elements of memory cells in an adjacent column or an adjacent row. The prior art stands on the position that the magnetization of the hard layer is inverted only by the combined magnetic field of magnetic fields induced by the currents flowing in the write line and the bit line and the magnetization of the hard layer is not inverted by the magnetic field induced by the current in only either the bit line or the write line.
However, when memory cells are disposed in high density and the intervals of adjacent memory cells are narrowed, the magnetic field induced by the current flowing in the write line and/or bit line also exerts an influence on an adjacent memory cell. Such a leakage magnetic field provides magnetic noise (magnetic field interference or magnetic disturbance) to a non-selected memory cell. Since a current of a predetermined magnitude flows in the bit line and the write line, such a situation that write data in a non-selected adjacent memory cell is inverted by such magnetic noise occurs.
In writing data of a plurality of bits, when simultaneously selecting adjacent memory cells, write currents have to be supplied to adjacent bit lines. In this case, if the logic levels of the write data are opposite, it is necessary to supply current to selected bit lines in the opposite directions. However, there may be a case that due to interference of the magnetic fields, the magnetic field of a desired intensity cannot be supplied to a selected memory cell and the data cannot be written accurately.
The prior art literature 1 does not consider the problem of erroneous writing caused by the magnetic noise on an adjacent memory cell and magnetic field interference at the time of parallel writing of multi-bit data at all.